It is of a very complicated nature, and consists of
numerous thin, slightly tortuous layers of a pale-coloured feldspathic
stone, often passing into an imperfect pitchstone, alternating with layers
formed of numberless little globules of two varieties of obsidian, and of
two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The
sphaerulites are either white and translucent, or dark brown and opaque;
the former are quite spherical, of small size, and distinctly radiated from
their centre. The dark brown sphaerulites are less perfectly round, and
vary in diameter from the twentieth to the thirtieth of an inch; when
broken they exhibit towards their centres, which are whitish, an obscure
radiating structure; two of them when united sometimes have only one
central point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united two
or three or many together into irregular groups, or more commonly into
layers, parallel to the stratification of the mass. This union in many
cases is so perfect, that the two sides of the layer thus formed, are quite
even; and these layers, as they become less brown and opaque, cannot be
distinguished from the alternating layers of the pale-coloured feldspathic
stone. The sphaerulites, when not united, are generally compressed in the
plane of the lamination of the mass; and in this same plane, they are often
marked internally, by zones of different shades of colour, and externally
by small ridges and furrows. In the upper part of Figure 6, the
sphaerulites with the parallel ridges and furrows are represented on an
enlarged scale, but they are not well executed; and in the lower part,
their usual manner of grouping is shown. In another specimen, a thin layer
formed of the brown sphaerulites closely united together, intersects, as
represented in Figure 7, a layer of similar composition; and after running
for a short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first intersected.
The small nodules also of obsidian are sometimes externally marked with
ridges and furrows, parallel to the lamination of the mass, but always less
plainly than the sphaerulites. These obsidian nodules are generally
angular, with their edges blunted: they are often impressed with the form
of the adjoining sphaerulites, than which they are always larger; the
separate nodules seldom appear to have drawn each other out by exerting a
mutually attractive force. Had I not found in some cases, a distinct centre
of attraction in these nodules of obsidian, I should have been led to have
considered them as residuary matter, left during the formation of the
pearlstone, in which they are embedded, and of the sphaerulitic globules.

The sphaerulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in sedimentary
deposits, that one is at once tempted to attribute to them an analogous
origin. They resemble ordinary concretions in the following respects: in
their external form,--in the union of two or three, or of several, into an
irregular mass, or into an even-sided layer,--in the occasional
intersection of one such layer by another, as in the case of chalk-flints,-
-in the presence of two or three kinds of nodules, often close together, in
the same basis,--in their fibrous, radiating structure, with occasional
hollows in their centres,--in the co-existence of a laminary,
concretionary, and radiating structure, as is so well developed in the
concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in
sedimentary deposits, it is known, are due to the separation from the
surrounding mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I have
endeavoured to discover whether obsidian and the sphaerulites (to which may
be added marekanite and pearlstone, both of them occurring in nodular
concretions in the trachytic series) differ in their constituent parts,
from the minerals generally composing trachytic rocks.